One existing control system for this kind of lifting device comprises a mechanism that lifts a load, a source for driving that drives the mechanism, a control portion that controls the source, and a portion for manipulation. The sensor that is provided in the portion for manipulation detects the force of an operator for holding up a load in the direction opposite to that of the pull of gravity when an operator holds the portion for manipulation and intends to lift the load. Then, the device amplifies its power for lifting in accord with the operator's force for holding it up. Thus it is lifted by both the force for holding up the load and the power for hoisting. The device controls the supply of air to a cylinder (i.e., a source for driving), so that the ratio of the power for lifting to the force for holding up the load is constantly or nearly constantly increased, as the force for holding up it is increased (see Japanese Patent Laid-open No. H11-147699).